100%(1)1 out of 1 people found this document helpful
This preview shows page 9 - 12 out of 56 pages.
11.1 Triangles120CADBEFFigure 11.5
11.2 The Parallel Postulate121We will prove that the sum of the angles of a triangle is a straight angle. Inthe approach that we follow, the following partial result is essential.Theorem 11.1.13.The sum of any two angles of a triangle is less than 180degrees.Proof.Consider an arbitrary triangleABCas depicted in Figure 11.7, and ex-tend the sideABbeyondAas shown.We will show that the sum of anglesCABandACBis less than a straight angle.AFBMCD